A physical hybrid automatic repeat request indicator channel is a downlink feedback channel in long term evolution (LTE) and is used by an Evolved Node B (eNB) to transmit acknowledgement and negative acknowledgement (ACK/NACK) messages to user equipments (UEs) regarding whether previously scheduled physical uplink shared channel (PUSCH) data has been successfully received by the eNB. In the current implementation of PHICH, multiple PHICH groups may be configured and each PHICH group consists of eight PHICHs. PHICHs in the same PHICH group share the same time-frequency resources. Three resource element groups (REGs) are allocated for one PHICH group. The REGs are distributed throughout the entire bandwidth to achieve frequency diversity. Each REG consists of four resource elements (REs) that are transmitted over adjacent sub-carriers. Each PHICH is assigned with one of eight orthogonal sequences and different PHICHs in a PHICH group are multiplexed in a code division multiplexing (CDM) manner in each REG. Because the wireless propagation channel is almost flat for symbols inside each REG, the orthogonality among different PHICHs is preserved even after travelling through a wireless channel.
However, due to the orthogonality requirement, a PHICH group is limited to eight bits, or eight PHICHs, since eight orthogonal sequences are defined for PHICH. Further, in current implementations, the sequences are orthogonal only in the real domain and thus high order modulations such as M-PSK (phase shift keying) or M-QAM (quadrature amplitude modulation) cannot be used to increase the capacity of the PHICH group, where M=2, 4, 8, etc.
If more than eight PHICH bits need to be transmitted, another PHICH group needs to be used, therefore utilizing more radio resources.